1. Field of the Invention
The present invention relates to a light modulation device suitable for optical display, light coupling, optical recording and light communication, and more particularly to a liquid crystal light modulation device which modulates a light by controlling the refraction index relative to the incident light by changing the orientation of a liquid crystal.
2. Description of the Prior Art
In a known light modulation device which uses a liquid crystal, a liquid crystal (Nematic liquid) crystal having a position dielectricity) is filled between orthogonally oriented transparent electrodes and the liquid crystal is helically arranged to form a TN (Twisted Nematic) cell. This device is inserted between two polarization plates having orthogonal polarization directions to each other so that in a static state, having no electric field applied between the transparent electrodes, a light flux transmits through the orthogonal polarization plates, and in a state in which an electric field higher than a threshold is applied between the transparent electrodes the liquid crystal is oriented in the direction of the electric field (normal to the planes of the transparent electrodes) and the light flux is blocked by the exit polarization plate and is not transmit therethrough. In another device, transparent substrates having parallel electrodes are arranged such that the electrode planes face each other and a liquid crystal is filled between the electrodes to form a homogeneously oriented device. This device is usually used as a light switch. The orientation of the liquid crystal is changed by applying an electric field to change a refraction index, and a light flux is directed to the substrate plane of the device at a predetermined angle so that the incident light is totally reflected or totally transmitted at an interface between the liquid crystal and the electrode.
Japanese Examined Patent Publication No. 3928/1978 discloses a light modulation device in which a liquid crystal is filled between substrates having a diffraction grating of periodic unevenness formed on a side facing the liquid crystal of at least one of the substrates, and the orientation of the liquid crystal is controlled to change a refraction condition to an incident light to modulate the light. This device utilizes light interference at the diffraction grating and provides a clear color by hue modulation. It can achieve various modulations by utilizing waveform selectivity of the diffraction grating.
However, in the prior art light modulation devices using the liquid crystal, including the light modulation device disclosed in Japanese Examined Patent Publication No. 3928/1978, conventional rubbing or oblique evaporation of SiO.sub.2 or MgF.sub.2 is used to orient the liquid crystal. Therefore, the degree of orientation of the liquid crystal is small, response time is slow and temperature stability is low. In the TN cell, the response time cannot be improved because of the thickness of the cell, the light utilization efficiency is low because of the use of the polarization plates and the contrast ratio is low. Therefore, it does not provide a satisfactory performance for the display device.